1. Field of the Invention
The present invention is generally directed to display devices and, more particularly, to active matrix liquid crystal display devices in which pixels (e.g., picture elements or picture cells) are formed by use of thin film transistors and pixel electrodes.
2. Description of the Invention
An active matrix liquid crystal display device includes a liquid crystal display unit on which a plurality of pixels are arranged in matrix form. Each individual pixel on the liquid crystal display unit is disposed in each of intersection regions defined by two adjacent scanning signal lines (gate signal lines) and two adjacent image signal lines (drain signal lines). The plurality of scanning signal lines extending in the row-direction (horizontal direction) are arrayed in the column-direction, while the plurality of image signal lines extending in the column-direction (vertical direction), intersecting the scanning signal lines, are arrayed in the row-direction.
The pixel is formed mainly of a liquid crystal in combination with a thin film transistor (TFT), a common transparent pixel electrode and a transparent pixel electrode which are disposed through the liquid crystal. The transparent pixel electrode and the thin film transistor are each provided for every pixel. The transparent pixel electrode is connected to a source electrode of the thin film transistor. A drain electrode of the thin film transistor is connected to the image signal lines, while a gate electrode is connected to the scanning signal lines.
A typical arrangement is such that unnecessary incident light emerging from a panel front surface is shielded by a light shielding film formed on the upper portion of TFT, and beams of backlight which are not required are shielded by the non-transparent gate electrode. In accordance with a variety of experiments performed, the present inventors have found that sufficient light shielding effects cannot be obtained by a TFT gate electrode of an ordinary size.
When the light strikes upon an amorphous semiconductor layer of the thin film transistor, electron-hole couplings are generated, thereby deteriorating OFF-characteristics of the transistor. Hence, it is required that the amorphous semiconductor layer be arranged so as not to undergo the irradiation of light as much as possible. The light for display is classified into two types: natural incident light (or light of a room lamp) emerging from the front surface of the liquid crystal display panel and incident backlight of a fluorescent lamp which emerges from the underside of the panel.
The above-described liquid crystal display device tends to increase in the size of a pixel thereof, as the liquid crystal display unit is correspondingly increased in configuration. For instance, the size of pixel of the conventional liquid crystal display unit was 0.2xc3x970.2 (mm2). However, the present inventors have developed a liquid crystal display device having a pixel size of 0.32xc3x970.32 (mm2).
In this type of liquid crystal display device, foreign substances such as dust or the like are intermixed in the liquid crystal display device in the manufacturing process, or the foreign substances are adhered to a mask for use with photolithography. If the foreign substances are present or intermixed in between the source electrode (or transparent pixel electrode) and the drain electrode of the thin film transistor, short-circuiting takes place between these electrodes, resulting in a so-called point defect in which the short-circuited pixel is deteriorated. If the foreign substances are likewise present or intermixed in between the source electrode (transparent pixel electrode) and the gate electrode of the thin film transistor, the same point defect is caused. From this phenomenon, the present inventors have found out such a problem that the point defect (e.g., a loss of pixel), inherent in the above-described liquid crystal display device, becomes conspicuous, as each individual pixel increases in size.
Incidentally, the arrangement that a configuration of the gate electrode is made larger than the semiconductor layer has already been known in Japanese Patent Laid-Open Publication No. 17962/1985. However, even when simply increasing the size of the gate electrode, a parasitic capacitance between the gate electrode and the source electrode also increases, and a DC component applied to the liquid crystal due to scanning signals is increased. In all, the undesirable results become so prevalent that utilization is difficult.
An example of an active matrix liquid crystal display device is described on, e.g., pp. 193 to 200 of NIKKEI ELECTRONICS issued on Dec. 15, 1986, published by Nikkei McGraw-Hill Co., Ltd.
The following listings are exemplary of the pixel dividing technique in the active matrix liquid crystal display device: Japanese Patent Laid-Open Publication Nos. 49994/1982, 78388/1984, 97322/1985 and 77886/1986.
It is a primary object of the present invention to provide a liquid crystal display device capable of reducing deterioration in OFF-characteristics of a TFT due to light incident on the TFT.
To this end, according to one aspect of the invention, there is provided a liquid crystal display device capable of improving the OFF-characteristics of the TFT and restraining a DC component applied to the liquid crystal.
According to another aspect of the invention, there is provided, in the liquid crystal display device, a technique capable of diminishing a point defect which causes deterioration of pixels on a liquid crystal display unit.
According to still another aspect of the invention, there is provided, in the liquid crystal display device, a technique capable of making it hard to visually perceive the point defect which appears on the liquid crystal display unit.
According to a further aspect of the invention, there is provided, in the liquid crystal display device, a technique capable of decreasing the point defect which causes the deterioration of the pixels on the liquid crystal display unit and also reducing black scattering appearing on the liquid crystal display unit thereof.
According to a still further aspect of the invention, there is provided, in the liquid crystal display device, a technique capable of accomplishing the above-described objects, decreasing a resistance value of scanning signal lines and reducing the point defect attributed to short-circuiting between a pixel electrode of the pixel and the scanning signal lines.
Another object of the invention is to provide a technique capable of reducing the black scattering and preventing disconnection of the electrodes of a holding (or storage) capacitance element for diminishing the black scattering.
Another object of the invention is to provide, in the liquid crystal display device, a technique capable of reducing the black scattering with a simple constitution.
Another object of the invention is to provide, in the liquid crystal display device, a technique capable of reducing the DC component applied to the liquid crystal of the liquid crystal display unit and diminishing the black scattering.
Another object of the invention is to provide, in the liquid crystal display device including color filters, a technique capable of reducing the point defect which appears on the liquid crystal display unit and ensuring positioning allowance dimensions with respect to each individual pixel on the liquid crystal display unit and each individual color filter for every color.
Another object of the invention is to provide, in the liquid crystal display device, a technique capable of diminishing the point defect appearing on the liquid crystal display unit and decreasing the probability that the point defect or a linear defect occurs on the liquid crystal display unit.
Another object of the invention is to provide, in the liquid crystal display device, a technique capable of diminishing the point defect appearing on the liquid crystal display unit and improving an area (an opening rate) of the pixel electrode of every pixel on the liquid crystal display unit.
Another object of the invention is to provide, in the liquid crystal display device, a technique capable of enhancing resolution of a color picture.
Another object of the invention is to provide a technique capable of accomplishing the above-described objects and reducing an area of wiring or eliminating a multilayered wiring structure.
Another object of the invention is to provide, in the liquid crystal display device, a technique capable of reducing the deterioration of connection between the thin film transistor and the pixel electrode.
Another object of the invention is to provide a liquid crystal display device capable of enhancing the contrast.
The principal features of the present invention are described as follows:
(1) A liquid crystal layer is sealed between a top-surface-side glass substrate (SUB2) on which a common electrode (ITO2) is formed and an underside glass substrate (SUB1) on which a pixel electrode (ITO1) and a TFT (TFT1) are formed (FIGS. 1 and 2). Viewed from the liquid crystal layer, a gate electrode (GT) of the TFT is disposed in close proximity to the underside substrate (SUB1), while a semiconductor layer (AS) is spaced away therefrom. The gate electrode (GT) has a large size which is sufficient to completely cover (when viewed from below) the semiconductor layer (AS).
According to this constitution, OFF-characteristics of the TFT can be improved, because the incident backlight passing through the underside substrate (SUB1) does not reach the semiconductor layer (AS) on account of its being shielded by the gate electrode (GT).
(2) The pixels are disposed in intersection regions defined by two scanning signal lines and two image signal lines. The thin film transistor of the pixel selected by one of two scanning signal lines is split into a plurality of segments. The thus divided thin film transistor is connected to a plurality of segments into which the pixel electrode is split. A holding capacitance element is constructed in such a way that the divided pixel electrodes serve as one electrode thereof, and the other of two scanning signal lines serves as the other electrode thereof by using it as a capacitance electrode line.
In this arrangement, only part of the divided portions of the pixel becomes the point defect. Otherwise, the point defect will spread over the entire pixel. It is therefore possible to diminish the point defect of the pixel and at the same time to improve a holding characteristic of a voltage applied to the liquid crystal due to the holding capacitance element, resulting in a drop in the amount of black scattering. Particularly, the divided pixels contribute to diminution in point defect derived from the short-circuit between the gate electrode and the source electrode or the drain electrode of the thin film transistor. Besides, the point defect attributed to the short-circuit between the pixel electrode and the other electrode of the holding capacitance element can be reduced. Consequently, the point defect created in part of the split portions of the pixels is small as compared with the area of the entire pixel, whereby it is hard to visually perceive the point defect.
Light shielding effects are enhanced by broadening the gate electrode. On the other hand, there arises a reverse effect in which the DC component applied to the liquid crystal becomes a problem because of an increase in overlapping parasitic capacitance between the source electrode and the drain electrode. This reverse effect can, however, be reduced by virtue of the holding capacitor.
(3) The scanning signal lines are composed of composite films obtained by superposing a plurality of conductive layers on each other. The gate electrode and the capacitance electrode line are each composed of single layered films each consisting of one conductive layer among the composite films.
Based on this construction, in addition to the above-described effects, it is feasible to decrease a resistance value of the scanning signal lines and reduce the point defect due to the short-circuit between the pixel electrode and the scanning signal lines.
(4) Formed between one electrode of the holding capacitance element and a dielectric film thereof is a base layer composed of a first conductive film and a second conductive film which is formed thereon and has a smaller specific resistance value and a smaller configuration than those of the first conductive film. The above-described one electrode is connected to the first conductive film exposed from the second conductive film of the base layer.
Owing to this arrangement, it is possible to minimize the disconnection of one electrode of the holding capacitance element, because one electrode of the holding capacitance element can surely be bonded along a stepped portion caused by the other electrode thereof.
(5) The capacitance electrode line of the first stage or the final stage is connected to the common pixel electrode of the pixel.
In this arrangement, the capacitance electrode line of the first stage or the final stage and part of the conductive layer of an outside extension wire may be formed into one united body, and the common pixel electrode is connected to the outside extension wire. The scanning signal lines can therefore be connected to the common pixel electrode with a simple constitution.
(6) The capacitance electrode line or the scanning signal line of the first stage is connected to the scanning signal line or the capacitance electrode line of the final stage.
Based on this arrangement, the scanning signal lines and the capacitance electrode lines are all connected to a vertical scanning circuit, and hence a DC offset system (a DC cancel system) may be adopted. As a result, the DC component applied to the liquid crystal can be reduced, thereby increasing a life span of the liquid crystal.
(7) In accordance with an embodiment III of the present invention which is illustrated in FIG. 15A, light shielding films 1 and 2 are provided to fill up gaps formed between the pixel electrodes ITO1 through ITO3.
In this embodiment III, the problem that the light such as backlight leaks out through the gaps between the pixel electrodes can be almost obviated.
(8) In accordance with an embodiment IV of the present invention, light shielding films 3 and 4 are electrically connected to an adjacent scanning line GL.
In the embodiment IV, capacitors may equivalently be formed between the light shielding films 1 and 2 (adjacent scanning line) and the respective divided pixel electrodes.
These and other objects, features and advantages of the invention will become more apparent on reading the following detailed description with reference to the accompanying drawings.